I have been try to work out what is happening on one of my PC's
http://setiathome.berkeley.edu/show_host_detail.php?hostid=7068185

The system is AMD X645 with 8Gb memory and GTX460 (1Gb)
I have just wiped the HD and reinstalled Win7, followed by Nvidia drivers 320.49

I put BOINC V7.0.64 (64bit) on and reattached to BAM which downloaded downloaded SETI and Engima. I then installed Lunatics 0.41 64bit installer.

Upon running MSI Afterburner I can see the GPU usage going up and down. ie the usage oscillation at 99% for 7 seconds then drops to 30% for one seconds and then back up to 99% and so on it goes up and down.

None of my other Nvidia PC's do this, is there a reason or a setting for this oscillation

You probably need to free a CPU core on that host to feed the GPU properly.

Maybe he already tried to free a core, but by mistake used the wrong %age box?

If you set the "Use at most ... %CPU time" control, it will run as described - all CPUs for 7 seconds on, 1 second off at 87.5%

Instead, set that value to 100%, and use the alternative "use at most ... % of the processors" - at 87.5%, that will keep 3 CPUs continuously occupied (on his quad core), and leave one free.

Not that it should be necessary, since he's running Cuda, not OpenCL.

Edit - there was a range of BOINC clients - v7.0.45 thru v7.2.0 - where David (inadvisedly IMHO) made CPU time throttling apply to GPU apps too. Unfortunately, this includes the currently-recommended v7.0.64 which Sean is using.

Also, perhaps check there aren't any crazy unnecessary motherboard utilities, or other background processes you don't want/need, running. Not that erratic usage is a problem, if expected due to using the PC, but annoying if youthink the system is clean & there is other stuff stealing the Bus (Been there ;) ).
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"It is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is the most adaptable to change."
Charles Darwin